Hydrodynamics of Solid Suspension and Distribution

Solid suspension requires the input of mechanical energy into the fluid-solid system by some mode of agitation. The input energy creates a turbulent flow field in which solid particles are lifted from the vessel base and subsequently dispersed and distributed throughout the liquid. Nienow (1985) discusses in some detail the complex hydrodynamic interactions between solid particles and the fluid in mechanically agitated vessels. Recent measurements (Guiraud et al 1997 Pettersson and Rasmuson, 1998) of the 3D velocity of both the fluid and the suspension confirm the complexity. 

Solids pickup from the vessel base is achieved by a combination of the drag and lift forces of the moving fluid on the solid particles and the bursts of turbulent eddies originating from the bulk flow in the vessel. This is clearly evident in 

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For small solid particles whose density is approximately equal to that of the liquid, once suspended they continue to move with the liquid. The suspension behaves like a single-phase liquid at low solid concentrations the mixing operation is more like blending than solid suspension. For heavier solid particles, their velocities will be different from that of the liquid. The drag force on the particles caused by the liquid motion must be sufficient and directed upward to counteract the tendency of the particles to settle by the action of gravity. 

The properties of both the liquid and the solid particles influence the fluid-particle hydrodynamics and thus the suspension. Also important are vessel geometry and agitation parameters. The important fluid and solid properties and operational parameters include  

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The structure of the cake formed and, consequently, its resistance to liquid flow depends on the properties of the solid particles and the liquid phase suspension, as well as on the conditions of filtration. Cake structure is first established by hydrodynamic factors (cake porosity, mean particle size, size distribution, and particle specific surface area and sphericity). It is also strongly influenced by some factors that can conditionally be denoted as physicochemical. These factors are ... 

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